Maybe is a sum because it was defined using sums. It's a monad because, from that definition, you can implement functions meeting the monad interface. "Monad" i

Maybe is a sum because it was defined using sums. It's a monad because, from that definition, you can implement functions meeting the monad interface. "Monad" is more like an adjective than a noun. > I wonder if there is an interesting interaction between those. There's a rather rich theory of polynomial endofunctors (i.e. generic types with one type parameter built from products, sums, and non-generic types), and I think monads figure prominently (they're somehow related to the initial algebras for such functors). So there's definitely a relationship there.